Mom's Genes May Explain Why Women Outlive Men

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An evolutionary "loophole" might explain why males of many
species live shorter lives than their female counterparts, a new
study finds.

The loophole lies in the mitochondria, the
energy-generating parts of our cells. The mitochondria have
their own DNA, separate from the DNA that resides in the nucleus
of the cell that we usually think of when we think of the genome.
In almost all species, the mitochondria DNA is passed down solely
from mother to child, without input from dad.

This direct line of inheritance may allow harmful mutations to
accumulate, according to a new study detailed today (Aug. 2) in
the journal Current Biology. Ordinarily, natural selection helps
keep harmful mutations to a minimum by ensuring they're not
passed down to offspring. But if a mitochondrial DNA mutation is
dangerous only to males and doesn't harm females, there's nothing
to stop mom from passing it to her daughters and sons.

"If a mitochondrial mutation pops up that is benign in females,
or a mutation pops up that is beneficial to females, this
mutation will slip through the gates of natural
selection and go through to the next generation," said study
researcher Damian Dowling, an evolutionary biologist at Monash
Univeristy in Australia.

The result: a load of mutations that don't harm females, but add
up to a
shorter life span for males.

Mother's Curse

Dowling and his colleague tested this idea — dubbed "Mother's
Curse" — in fruit flies (Drosophila melanogaster). They
took flies with standardized nuclear genomes, meaning all had the
same cellular DNA, and inserted mitochondrial DNA from 13
different fruit-fly populations around the world. [ Global
Life Expectancy (Infographic) ]

"The only genetic difference across the strains of flies lay in
the origin of the mitochondria," Dowling told LiveScience.

The researchers then recorded how long each strain of flies
lived. Their findings revealed a big difference for males, but
not for females.

"There was a lot of variation in terms of
male longevity and male aging, but almost no variation in the
female parameters of aging," Dowling said. "This provides very
strong evidence that there are lots of mutations within the
mitochondrial genome that are having an effect on male aging, but
are having no effect whatsoever on female aging."

Explaining the gender gap

This finding bolsters the Mother's Curse hypothesis, Dowling
said. And the results suggest that the age gap between males and
females does not come down to just a few genes.

"In some ways this is bad news for medical biologists, because
we're not looking for the mutation that causes early
male aging, we're actually dealing with a whole lot of mutations
within this genome that are teaming up to shorten male life
span," Dowling said. [ 5
Reasons Aging Is Awesome ]

The genetic inheritance of mitochondrial DNA is the same across
species, so Dowling said he'd expect to see the same results in
human males. There is speculation that
women outlive men because men are generally bigger
risk-takers or perhaps because testosterone, a hormone men have
more of, has deleterious effects on life span, he said. But
insects don't have testosterone or a tendency to drive too fast
while not wearing a seatbelt, making them a good place to start
looking for genetic underpinnings to the gender gap.

Males may not be entirely doomed, however, as evidenced by the
fact that
they haven't gone extinct yet. It's possible that the nuclear
genome — the DNA we inherit from both of our parents — might be
compensating for the mitochondrial handicap in men. In other
words, men whose genomes can counteract the nasty effects of
mitochondrial mutations might do better and pass on their genes
more effectively.